Organic light-emitting diode (OLED) display devices typically include a substrate having one or more OLED light-emitting elements including a first electrode formed thereon, one or more OLED light-emitting layers located over the first electrode, and a second electrode located over the OLED light-emitting layers, and an encapsulating cover located over the second electrode, affixed to the substrate, and forming a cavity between the inside of the encapsulating cover and the second electrode. Such an OLED device may be top-emitting, where the light-emitting elements are intended to emit through the cover, and/or bottom-emitting, where the light-emitting elements are intended to emit through the substrate. Accordingly, in the case of a bottom-emitting OLED device, the substrate and first electrode must be largely transparent, and in the case of a top-emitting OLED device, the cover and second electrode must be largely transparent.
Referring to FIG. 3, a top-emitter OLED device discussed in the prior art includes a substrate 10 on which is deposited one or more first electrodes 20 separated by insulators 28, one or more organic layers 22 comprising the OLED (at least one of which is emissive when a current is passed through the layers) and a second, common electrode 24. An encapsulating cover 12 is affixed to the substrate 10 and seals the organic layers 22 from the environment. A cavity 14 exists between the second electrode 24 and the cover 12 and is usually filled with an inert gas. This cavity is typically from 10 microns to 50 microns wide, much thicker than a typical thin-film layer in an OLED device. Desiccant materials designed to protect the OLED materials may be located in the cavity, either above the light emissive area in the OLED device or around the periphery of the light emissive area in the OLED device. Thin-film protective layers (not shown) may also be deposited over the second electrode 24 and are employed to protect the second electrode 24. In this top-emitter configuration, light 26 is emitted through the cover so that the cover 12 and second electrode 24 must be transparent while the substrate 10 and the electrode 20 may be opaque or reflective.
Referring to FIG. 4, a commercially available bottom-emitter OLED device includes a substrate 10 on which is deposited one or more first electrodes 20, one or more organic layers 22 (at least one of which is emissive when a current is passed through the layers) and a second, common electrode 24. A cover 12 is affixed to the substrate and seals the OLED materials from the environment. A cavity 14 exists between the second electrode 24 and the cover 12 and is usually filled with an inert gas and may include desiccant materials. In this bottom-emitter configuration light 26 is emitted through the substrate 10 so that the substrate 10 and first electrode 20 must be transparent while the cover 12 and the second electrode 24 may be opaque or reflective.
A variety of materials may be used to construct suitable substrates and encapsulating covers for OLED devices and to fill the cavity between the second electrode and the cover. The desirable material properties and/or characteristics of an OLED substrate and cover include low cost, very flat surface, low coefficient of thermal expansion (CTE), high strength and stability under a variety of environmental stresses, and electrically non-conductive (or able to be coated with electrically non-conductive material). The material used most often for such substrates is glass, typically borosilicate glass, because it is transparent, very stable, can be made at low-cost, and has a very smooth surface suitable for the deposition and processing of semiconductor and organic materials. Other substrate materials have been described in the art, for example ceramics, plastics, and metals such as stainless steel (see U.S. Pat. No. 6,641,933 B1 to Yamazaki et al entitled “Light-emitting EL display device”). Because the OLED materials are very sensitive to moisture, the cavity between the second electrode and the cover is often provided with desiccant materials and the cover is carefully sealed to the substrate (see, e.g., US 20030203700 A1 entitled “Encapsulating OLED devices with transparent cover” by Clark published 2003 Oct. 30). Inert gases may be employed to fill the cavity; alternatively, it is known to provide polymer material to fill the cavity (see, for example, US 20030143423 A1 entitled “Encapsulation of organic electronic devices using adsorbent loaded adhesives” by McCormick, et al published 2003 Jul. 31).
JP 10-275681 discloses an organic electroluminescent light source having a light emitting element with a relatively thick cathode and a surrounding conforming protecting resin layer to provide high heat conductivity. However, as described for this arrangement, there is no disclosure of use of such surrounding protecting layer to adhere a separate encapsulating cover to the organic electroluminescent light source. The use of conforming resin protection layer, however, typically is inadequate itself to provide desired environmental protection to the organic electroluminescent materials. Further, there is no disclosure of the need to spread heat between an active light emitting element and an inactive light emitting element in a device comprising a plurality of light emitting elements, in order to reduce differential aging of such light emitting elements.
Organic light-emitting diodes can generate efficient, high-brightness displays. However, heat generated during the operation of the display in high-brightness modes can limit the lifetime of the display, since the light-emitting materials within an OLED display degrade more rapidly when used at higher temperatures. While it is important to maintain the overall brightness of an OLED display, it is even more important to avoid localized degradation within a display. The human visual system is acutely sensitive to differences in brightness in a display. Hence, differences in uniformity are readily noticed by a user. Such localized differences in uniformity in an OLED display may occur as a consequence of displaying static patterns on the display, for example, graphic user interfaces often display bright icons in a static location. Such local patterns will not only cause local aging in an OLED display, but will also create local hot spots in the display, further degrading the light-emitting elements in the local pattern. Glass and plastic supports, the use of which is advantageous in view of their relative electrical non-conductivity, may not be sufficiently thermally conductive to provide a uniform temperature across the substrate when the display is in operation. Hence, improved thermal management techniques may significantly improve the life expectancy of an organic display device.
One method of removing heat from an organic light emitting display device is described in U.S. Pat. No. 6,265,820, entitled, “Heat removal system for use in organic light emitting diode displays having high brightness.” The '820 patent describes a heat removal system for use in organic light emitting diode displays. The heat removal assembly includes a heat dissipating assembly for dissipating heat from the organic light emitting device, a heat transfer assembly for transferring heat from the top organic light emitting device to the heat dissipating assembly and a cooling assembly for cooling the organic light emitting display device. While the system of the '820 patent in one embodiment provides a thermally conductive intermediate material positioned between the organic light emitting device and a sealed backplate, the use of specific materials suggested (metallic layers or non-metallic thermal paste) do not provide adhesion or are difficult to assemble in OLED devices. Moreover, the structure described in the '820 patent is complex, requiring multiple layers.
U.S. Pat. No. 6,480,389 to Shie et al entitled “Heat dissipation structure for solid-state light emitting device package” describes a heat dissipation structure for cooling inorganic LEDs and characterized by having a heat dissipating fluidic coolant filled in a hermetically sealed housing where at least one LED chip mounted on a metallic substrate within a metallic wall erected from the metallic substrate. Such an arrangement is complex, requires fluids, and is not suitable for area emitters such as OLEDs.
US 2004/0004436 A1 entitled “Electroluminescent display device” by Yoneda published Jan. 8, 2004, describes an organic EL panel having a device glass substrate provided with an organic EL element on a surface thereof, a sealing glass substrate attached to the device glass substrate, a desiccant layer formed on a surface of the sealing glass substrate, and nonadhesive (e.g., metal) spacers disposed between a cathode of the organic EL element and a desiccant layer. A heat-conductive layer can be formed by vapor-depositing or sputtering a metal layer such as a Cr layer or an Al layer that inhibits damaging the organic EL element and increases a heat dissipating ability, thereby inhibiting aging caused by heat.
U.S. Pat. No. 6,633,123 B2 entitled “Organic electroluminescence device with an improved heat radiation structure” issued 2003 Oct. 14 provides an organic electroluminescence device including a base structure and at least an organic electroluminescence device structure over the base structure, wherein the base structure includes a substrate made of a plastic material, and at least a heat radiation layer which is higher in heat conductivity than the substrate.
U.S. Pat. No. 5,821,692 A entitled “Organic electroluminescent device hermetic encapsulation package” issued 1998 Oct. 13 describes an organic electroluminescent device array encapsulating package including an organic electroluminescent device on a supporting substrate. A cover having a rim engaging the supporting substrate is spaced from and hermetically encloses the organic electroluminescent device. A dielectric liquid having benign chemical properties fills the space between the cover and the organic electroluminescent device, providing both an efficient medium for heat transmission and an effective barrier to oxygen and moisture. Similarly, JP11195484 A entitled “Organic EL Element” by Yasukawa et al. published 1999 Jul. 21 describes an organic EL element equipped with an organic EL structural body laminated on a substrate, and a sealing plate arranged on the organic EL structural body with a predetermined gap, where a sealing substance having heat conductivity of 1.1×10−1 W·m−1·K−1 or more, and viscosity of 0.5 to 200 cP at 25° C. is filled in the sealed space. While such dielectric liquids can be useful, applicant's experience with such materials is that they are difficult to use in manufacturing. Moreover, if the substrate and cover of an OLED device have significantly differing coefficients of thermal expansion and the OLED device is heated, the materials cited do not provide additional adhesion that may be necessary to prevent the seal between the cover and the substrate from breaking.
JP2003100447 A entitled “Organic Electroluminescence Equipment” by Hashimoto et al. published 2003 Apr. 4 describes a high sealing resin layer and a high heat-conductivity resin layer formed in the gap of a glass substrate and a sealing substrate in the perimeter part of the sealing substrate. Such a layer does not assist in removing or spreading heat from the emissive areas of the OLED device, and also does not improve adhesion to the cover in such emissive areas.
Heat sinks are also well known in the integrated circuit industry and are applied to cooling large integrated circuits. Such sinks typically are thick and are unsuitable for displays in which limiting the thickness of the display is an important goal. Moreover, heat sinks do not improve the thermal conductivity of an OLED device itself.
It is therefore an object of the present invention to provide a more uniform distribution of heat within an OLED display to improve the removal of heat from an OLED display device thereby increasing the lifetime of the display, and to improve adhesion of an encapsulating cover in an OLED device.